3GPP-LTE (3rd Generation Partnership Project Radio Access Network Long Term Evolution, hereinafter referred to as “LTE”) adopts OFDMA (Orthogonal Frequency Division Multiple Access) as a downlink communication scheme, and SC-FDMA (Single Carrier Frequency Division Multiple Access) as an uplink communication scheme (e.g., see non-patent literatures 1, 2 and 3).
In LTE, a radio communication base station apparatus (hereinafter abbreviated as “base station”) communicates with radio communication terminal apparatuses (hereinafter abbreviated as “terminals”) by allocating resource blocks (RBs) in a system band to terminals, per time unit referred to as a subframe. In addition, the base station transmits to the terminals the control information (resource allocation information) to notify the terminals of the result of resource allocation of downlink data and uplink data. This control information is transmitted to the terminals using downlink control channels such as PDCCHs (Physical Downlink Control Channels). Here, according to, for example, an allocation number of terminals, the base station controls the amount of resources used in transmission of the PDCCHs, that is, the number of OFDM symbols on a subframe unit basis. To be more specific, the base station transmits to the terminals using a PCFICH (Physical Control Format Indicator Channel), the CFI (Control Format Indicator), which is the information indicating the number of OFDM symbols capable of being used in transmission of the PDCCHs in the first OFDM symbols of the subframes. Each of terminals receives the PDCCH in accordance with the CFI detected from the received PCFICH. Here, each PDCCH occupies a resource formed of one or a plurality of consecutive CCEs (Control Channel Elements). In LTE, according to the number of information bits of the control information or the channel state of the terminal, one of 1, 2, 4, and 8 is selected as the number of CCEs occupied by the PDCCH (the number of linked CCEs: CCE aggregation level). Here, LTE supports the frequency band with the maximum width of 20 MHz as a system bandwidth.
In addition, the base station simultaneously transmits a plurality of PDCCHs to allocate a plurality of terminals to one subframe. At this time, in order to identify the destination terminal of each of the PDCCHs, the base station includes a CRC bit masked (or scrambled) with the ID of the destination terminal in the PDCCH for transmission. Then, the terminal detects the PDCCH addressed to the terminal by performing blind decoding on a plurality of PDCCHs which may be addressed to the terminal, by demasking (or descrambling) CRC bits using its own terminal ID.
Furthermore, studies have been underway for a technique to limit the CCE to be the target of blind decoding every terminal, for the purpose of reducing the number of blind decoding attempts at the terminal. This technique limits the CCE area (hereinafter, referred to as “search space”) to be the target of the blind decoding every terminal. In LTE, the search space is randomly formed every terminal, and the number of CCEs forming the search space is defined every CCE aggregation level of the PDCCH. For example, for CCE aggregation levels 1, 2, 4, and 8, the numbers of CCEs forming the search spaces—that is, the numbers of CCEs to be the targets of the blind decoding—are limited to six candidates (6 (=1×6) CCEs), six candidates (12 (=2×6) CCEs), two candidates (8 (=4×2) CCEs), and two candidates (16 (=8×2) CCEs), respectively. Thus, each terminal needs to perform blind decoding only on the CCEs in the search space allocated to the terminal, thus making it possible to reduce the number of blind decoding attempts. Here, the search space of each terminal is configured using the terminal ID of each terminal and a hash function for randomization.
Also, LTE adopts ARQ (Automatic Repeat reQuest) for downlink data from the base station to the terminals. That is, each of the terminals sends a response signal indicating the error detection result of downlink data to the base station as a feedback. The terminal performs a CRC on the downlink data, and then, transmits a response signal (that is, ACK/NACK signal) indicating an ACK (Acknowledgement) in case of CRC=OK (no error) or a NACK (Negative Acknowledgement) in case of CRC=NG (error exists) as a feed back to the base station. When the response signal transmitted as a feedback indicates the NACK, the base station transmits retransmission data to the terminal. Moreover, in LTE, the control technique for retransmitting data, referred to as HARQ (Hybrid ARQ), which combines error correction coding and ARQ, has been examined. In HARQ, when receiving retransmitted data, the terminal can improve reception quality at the terminal side by combining the retransmitted data and the previously-received data including an error.
Moreover, standardization of 3GPP LTE-Advanced (hereinafter referred to as “LTE-A”) to realize faster communication than the LTE has been started. In LTE-A, in order to realize the downlink transmission speed equal to or higher than the maximum 1 Gbps and the uplink transmission speed equal to or higher than the maximum 500 Mbps, it is expected to introduce base stations and terminals (hereinafter referred to as “LTE-A terminals”) capable of communicating with each other at the wideband frequency equal to or higher than 40 MHz. In addition, an LTE-Advanced system is required to accommodate not only LTE-A terminals but also the terminals supporting an LTE system (hereinafter referred to as “LTE terminals”).
In LTE-A, the carrier aggregation scheme whereby communication is performed by aggregating a plurality of frequency bands has been proposed to realize wideband communication of 40 MHz or above (e.g., see non-patent literature 1). For example, the frequency band having a width of 20 MHz is defined as the base unit (hereinafter referred to as “component carrier (CC)”) of communication bands. Thus, LTE-A realizes the system bandwidth of 40 MHz by aggregating two component carriers. Also, a single component carrier accommodates both an LTE terminal and an LTE-A terminal. Additionally, in the following explanation, the component carrier in an uplink is referred to as “uplink component carrier”, and the component carrier in a downlink is referred to as “downlink component carrier.”
While it has been studied to support the carrier aggregation by at least five component carriers in the LTE-A system, the number of actually used component carriers differs every terminal according to, for example, a required transmission rate and the reception capability of each terminal with the number of component carriers. Here, which component carrier to be used is configured every terminal. The configured component carrier is referred to as “UE CC set.” The UE CC set is semi-statically controlled by the required transmission rate of the terminal.
In LTE-A, as a method to notify terminals of the resource allocation information of each component carrier from a base station, it has been discussed to allocate data of different component carriers by a PDCCH transmitted using a certain component carrier (e.g., see non-patent literature 4). In particular, studies have been underway to indicate the component carrier which is the allocation target of the PDCCH by using a carrier indicator (CI) in the PDCCH. That is, the CI labels each component carrier. The CI is transmitted in a field inside of the PDCCH, referred to as “carrier indicator field (CIF).”
Also, it has been considered to report the CIF value of the component carrier which is the allocation target, in addition to the CI in the CIF (e.g., see non-patent literature 5).
Also, the above non-patent literature 4 discloses the correspondence between a CI value (that is, a code point) and the CC number indicated by the CI value. That is, when the same CC as the CC which has transmitted a PDCCH is allocated, CI=1 (when CI starts from 1) is allocated. CI values are associated in ascending frequency order with other CCs. For example, as illustrated in FIG. 1B, when there are three CCs (CC1, CC2, and CC3 in ascending frequency order) and all three CCs are configured to a terminal (that is, when a UE CC set includes CC1, CC2, and CC3), in the PDCCH transmitted in CC2, CI=1 indicates data allocation of CC2, CI=2 indicates the data allocation of CC1, and CI=3 indicates the data allocation of CC3. Meanwhile, as illustrated in FIG. 1A, two out of three CCs are configured to the terminal (for example, when a UE CC set includes CC2 and CC3), CI=1 indicates the data allocation of CC2 and CI=2 indicates the data allocation of CC3. In this case, every time the CC configuration of each terminal (that is, the UE CC set) is changed, the correspondence between the CIs and the CC numbers varies, the CIs being other than the CI allocating the same CC. In the above example, when CC1 is added to the UE CC set in the terminal for which CC2 and CC3 are configured, the code point of the CI allocating CC3 varies before and after adding the CC.
Here, use of RRC signaling described in non-patent literature 6 to change the UE CC set (that is, addition and deletion of a CC), for example, has been considered. To be more specific, an RRC connection reconfiguration procedure is used to change the UE CC set. In case of changing a UE CC set, a base station firstly transmits an RRC connection reconfiguration message to a terminal to notify the terminal of the change. The terminal receiving this message changes its configuration, and then, after the change is completed, and sends an RRC connection reconfiguration complete message to the base station. By receiving the RRC connection reconfiguration complete message, the base station learns that the configuration change has been correctly made in the terminal. Here, it normally takes several 10 to 100 ms to communicate these messages with each other.